1. Field of the Invention
The present invention relates to a focus measurement method. More particularly, the present invention relates to a method of finding the best focus of an exposure process by using a specially designed pattern mask.
2. Description of the Prior Art
Photolithography is a process of transferring the patterns on a mask into a photoresist over a wafer. In order to completely and exactly transfer patterns into the photoresist, the patterns projected onto the photoresist by the exposure machine should have a certain depth of focus (DOF) such that the whole photoresist, both near the surface side of the photoresist and near the wafer side, has the same focus. With the decreasing depth of focus of modern high NA lithographic tools, accurate and reliable determination of the best focus, tilting, and field curvature has become increasingly important.
Conventionally, focus exposure matrix (FEM) is exposed with dot reticle, then the best focus is found using the naked eye. The foregoing method is fast, but it is easy to make a wrong decision. This results in low accuracy (error range is xc2x10.1 xcexcm) This method is presently used to correct the exposure machine daily.
Another method of measuring the best focus is using a focus exposure matrix with line patterns to carry out the photolithography process, then measuring linewidth by SEM. However, due to improvements in photoresist material, the photoresist maintains almost the same linewidth over a wide range of defocus. Therefore, it is almost impossible to determine the best focus by SEM measurement of photoresist linewidth.
Therefore, the present invention provides a simple method to easily find the best focus with high accuracy.
Furthermore, the present invention provides a method to detect tilting or field curvature for each product.
Moreover, the present invention provides a mask, which can be used to monitor focus, tilting and field curvature everyday.
The present invention provides a focus measurement method, comprising: providing a mask having a pattern composed of a first rectangle and a second rectangle located inside the first rectangle, the first rectangle and the second rectangle having the same center of gravity, each of the first and second rectangles having four bars, each bar having several columns of different pattern width but the same length, wherein adjoining patterns in the same column are separated by a first distance, and adjoining patterns belonging to different columns are separated by a second distance, wherein, for corresponding bars of the first and second rectangles, the widths of the patterns belonging to different columns decrease in a first direction in the first rectangle, the widths of the patterns belonging to different columns increase in the first direction in the second rectangle; performing an over-exposure process with a variety of focus settings in a photoresist layer on a wafer by using the mask; measuring an overlay shift of the center of gravity of the first and second rectangles in the photoresist under the different focus settings; and fitting the function of the overlay shift to the focus setting by a second order polynomial equation and getting a fitting curve, then taking the derivative of the fitting curve so as to obtain the best focus, which is located at the point where the derivative is zero.
According to one embodiment of the present invention, each bar at least comprises two columns with two different pattern widths. Preferably, however, each bar comprises four columns with four different pattern widths. Exposure energy used in the over-exposure process should be sufficient to exceed the resolution of the holes. Preferably, exposure energy used in the over-exposure process is four times the threshold energy. Furthermore, seven points of the overlay shift can be used to obtain the fitting curve.